The present invention is directed to a method of producing nickel alloys having improved resistance to stress corrosion cracking. In particular, the present invention is directed to a method of fabricating NiCrFe alloys which exhibited excellent resistance to stress corrosion cracking in deaerated primary water (i.e., low oxygen content water used in nuclear reactors).
The problem of stress corrosion in deaerated primary water has existed for some time. Accordingly, the literature is replete with various approaches which attempt to solve this problem.
Initially, researches in this area looked at the wealth of literature accumulated over the years in coping with stress corrosion cracking of austenitic nickel-chromium stainless steels in chloride environments. However, this avenue of exploration was not successful because stress corrosion cracking of the stainless steels in chloride solution is primarily transgranular in nature whereas NiCrFe stress corrosion in deaerated water is intergranular.
Another approach to the stress corrosion cracking problem centered on the use of materials of very high purity, i.e., the use of extremely pure nickel, chromium and iron and not much else. This pursuit appeals more to theoretical curiosity since it is basically impractical from a commercial viewpoint. Not only is it expensive to use such materials but good commercial practice requires the use of various other elements to provide necessary deoxidizing and malleabilizing attributes and to provide, for example, good forging practice. Moreover, it is not unlikely that the mechanical properties of such alloys would be inferior.
Although the above discussed approaches did not solve the problem of SCC in NiCrFe alloy, they did serve to focus greater attention on appreciating the significance of the operating environmental conditions which could lead to the type of corrosive attack in question and, also, understanding the ostensible peculiarities or behavior of nickel-chromium-iron alloys upon exposure to such conditions. Perhaps it should be mentioned that high purity water as contemplated herein contains a total solids content of less than one part per million (p.p.m.) by weight and has been distilled and/or deionized or otherwise treated such that it will manifest a specific resistance of about 500,000 ohm-cm or higher.
Certain environmental conditions have been established which either promote or are causative of inducing or creating a propensity for detrimental intergranular stress-corrosion cracking to occur in nickel-chromium-iron alloys. Aerated high-purity water (in combination with the surface condition of the alloys) is one such condition and temperature is another. Normally, high-purity water is devoid of oxygen, and it is believed that the usual absence thereof, has been responsible, to a considerable degree, for the lack of intergranular stress-corrosion cracking of nickel-chromium-iron alloys. But the possibilities of oxygen contamination are indeed more than sufficient to warrant the necessity of finding alloys which afford a markedly higher degree of resistance to such attack. As to temperature, if the temperature of the water is at about room temperature, stress-corrosion attack does not appear to be much of a problem. But, in commercial operation the temperature of the high-purity water is normally above room temperature and is commonly over 300.degree. F. e.g., about 450.degree. or 500.degree. F., to about 660.degree. F. and it is at such temperatures, particularly at the higher temperatures, where the occurrence of intergranular stress-corrosion attack is most likely.
U.S. Pat. No. 3,645,726 to Copson et al, proposed a solution to the SCC problem by limiting the composition of the NiCrFe alloy. While this solution has been successful to some degree it too is lacking for two reasons. First, while the proposed NiCrFe alloys are somewhat more resistant to SCC they are still subject to a high degree of uncertainty. That is, the incidence of SCC with these alloys is still unacceptably high. Accordingly, even these alloys must be subjected to severe testing procedures (i.e., evaluation) prior to installation in nuclear reactor facilities. (See U.S. Pat. No. 4,300,890 to Steeves et al relating to SCC evaluation techniques). Secondly, the composition limits on the NiCrFe alloy severely limit one's choices with regard to materials. The present invention is directed to a process of producing NiCrFe alloy processing the required SCC resistance properties needed for use in a nuclear facility environment. To this end it has been observed, during evaluation procedures (See U.S. Pat. No. 4,300,890), that NiCrFe alloys which do possess adequate resistance to SCC have carbide precipitated primarily in their grain boundaries. Accordingly, applicants' invention is directed to a method of producing NiCrFe possessing the required carbide grain boundary precipitation.